diff --git a/packages/shared-schema/src/migrations/lww-replacement-barrier-v2-to-v3.ts b/packages/shared-schema/src/migrations/lww-replacement-barrier-v2-to-v3.ts
index 6a4fe6c388..b179dfffd5 100644
--- a/packages/shared-schema/src/migrations/lww-replacement-barrier-v2-to-v3.ts
+++ b/packages/shared-schema/src/migrations/lww-replacement-barrier-v2-to-v3.ts
@@ -1,7 +1,11 @@
 import type { SchemaMigration } from '../migration.types';
 
+const isRecord = (value: unknown): value is Record<string, unknown> =>
+  typeof value === 'object' && value !== null && !Array.isArray(value);
+
 /**
- * Compatibility barrier for synthetic LWW replacement payloads.
+ * Compatibility barrier for synthetic LWW replacement payloads, plus an idle
+ * config backfill.
  *
  * Schema v3 introduces replacement-mode LWW envelopes whose omitted fields
  * are intentionally cleared. NOTE: this stamp does NOT stop released clients.
@@ -13,11 +17,45 @@ import type { SchemaMigration } from '../migration.types';
  * post-v18.14.0 receivers only, which block any newer-schema op outright. See the
  * bump policy in schema-version.ts before relying on a version fence.
  * Historical v2 operations retain their patch semantics when migrated.
+ *
+ * State transform: v2 snapshots (v18.14 and earlier) can lack
+ * `globalConfig.idle.isSuppressIdleDuringFocusMode`, which #8965 added as a
+ * required field WITHOUT its own schema bump. The v4 state validator requires
+ * it, and the migration path validates the raw snapshot BEFORE the loadAllData
+ * reducer can fill its default — so on upgrade the whole snapshot fails
+ * validation and hydration aborts. Backfill the opt-in default (false) here to
+ * match DEFAULT_GLOBAL_CONFIG.idle, but only when the field is not already a
+ * boolean so an existing user choice is never clobbered. A missing `idle`
+ * section is left to the reducer's own default merge.
+ *
+ * The hardcoded `false` is intentionally decoupled from DEFAULT_GLOBAL_CONFIG:
+ * a migration must freeze the historical default even if the live default later
+ * changes (and shared-schema cannot import app code).
  */
 export const LwwReplacementBarrierMigration_v2v3: SchemaMigration = {
   fromVersion: 2,
   toVersion: 3,
-  description: 'Gate replacement-mode LWW operations from older clients.',
-  migrateState: (state: unknown): unknown => state,
+  description:
+    'Gate replacement-mode LWW operations; backfill idle.isSuppressIdleDuringFocusMode default.',
+  migrateState: (state: unknown): unknown => {
+    if (!isRecord(state)) {
+      return state;
+    }
+    const globalConfig = state.globalConfig;
+    if (!isRecord(globalConfig)) {
+      return state;
+    }
+    const idle = globalConfig.idle;
+    if (!isRecord(idle) || typeof idle.isSuppressIdleDuringFocusMode === 'boolean') {
+      return state;
+    }
+    return {
+      ...state,
+      globalConfig: {
+        ...globalConfig,
+        idle: { ...idle, isSuppressIdleDuringFocusMode: false },
+      },
+    };
+  },
   requiresOperationMigration: false,
 };
diff --git a/packages/shared-schema/tests/migrations/lww-replacement-barrier-v2-to-v3.spec.ts b/packages/shared-schema/tests/migrations/lww-replacement-barrier-v2-to-v3.spec.ts
index 85ec181921..f5b7fc4470 100644
--- a/packages/shared-schema/tests/migrations/lww-replacement-barrier-v2-to-v3.spec.ts
+++ b/packages/shared-schema/tests/migrations/lww-replacement-barrier-v2-to-v3.spec.ts
@@ -11,7 +11,7 @@ describe('LWW replacement compatibility barrier v2 -> v3', () => {
     expect(LwwReplacementBarrierMigration_v2v3.toVersion).toBe(3);
   });
 
-  it('leaves v2 state data unchanged', () => {
+  it('leaves state without a globalConfig unchanged', () => {
     const state = { task: { ids: ['task-1'] } };
 
     const result = migrateState(state, 2, 3);
@@ -20,6 +20,81 @@ describe('LWW replacement compatibility barrier v2 -> v3', () => {
     expect(result.data).toBe(state);
   });
 
+  it('backfills the missing isSuppressIdleDuringFocusMode default (#8965)', () => {
+    const state = {
+      task: { ids: [] },
+      globalConfig: {
+        idle: {
+          isEnableIdleTimeTracking: true,
+          isOnlyOpenIdleWhenCurrentTask: false,
+          minIdleTime: 5 * 60 * 1000,
+          // isSuppressIdleDuringFocusMode absent (v18.14 snapshot shape)
+        },
+      },
+    };
+
+    const result = migrateState(state, 2, 3);
+
+    expect(result.success).toBe(true);
+    const idle = (result.data as { globalConfig: { idle: Record<string, unknown> } })
+      .globalConfig.idle;
+    expect(idle.isSuppressIdleDuringFocusMode).toBe(false);
+    // Existing fields preserved
+    expect(idle.isEnableIdleTimeTracking).toBe(true);
+    expect(idle.minIdleTime).toBe(5 * 60 * 1000);
+  });
+
+  it('never clobbers an existing boolean isSuppressIdleDuringFocusMode value', () => {
+    const state = {
+      globalConfig: {
+        idle: {
+          isEnableIdleTimeTracking: true,
+          isOnlyOpenIdleWhenCurrentTask: false,
+          minIdleTime: 5 * 60 * 1000,
+          isSuppressIdleDuringFocusMode: true,
+        },
+      },
+    };
+
+    const result = migrateState(state, 2, 3);
+
+    expect(result.success).toBe(true);
+    // Identity when the field is already a boolean — same reference, user choice kept
+    expect(result.data).toBe(state);
+  });
+
+  it('backfills when the field is present but not a boolean (null/undefined)', () => {
+    // A `null`/`undefined` value would otherwise survive the reducer's
+    // `{ ...DEFAULT, ...idle }` merge and keep failing validation, so the guard
+    // must treat non-boolean the same as absent.
+    const state = {
+      globalConfig: {
+        idle: {
+          isEnableIdleTimeTracking: true,
+          isOnlyOpenIdleWhenCurrentTask: false,
+          minIdleTime: 5 * 60 * 1000,
+          isSuppressIdleDuringFocusMode: null,
+        },
+      },
+    };
+
+    const result = migrateState(state, 2, 3);
+
+    expect(result.success).toBe(true);
+    const idle = (result.data as { globalConfig: { idle: Record<string, unknown> } })
+      .globalConfig.idle;
+    expect(idle.isSuppressIdleDuringFocusMode).toBe(false);
+  });
+
+  it('leaves state whose globalConfig has no idle section unchanged', () => {
+    const state = { globalConfig: { misc: {} } };
+
+    const result = migrateState(state, 2, 3);
+
+    expect(result.success).toBe(true);
+    expect(result.data).toBe(state);
+  });
+
   it('preserves historical v2 operation semantics while stamping schema v3', () => {
     const operation: OperationLike = {
       id: 'legacy-lww',
diff --git a/src/app/op-log/persistence/operation-log-snapshot.service.spec.ts b/src/app/op-log/persistence/operation-log-snapshot.service.spec.ts
index de40323c99..fb3b5d8d65 100644
--- a/src/app/op-log/persistence/operation-log-snapshot.service.spec.ts
+++ b/src/app/op-log/persistence/operation-log-snapshot.service.spec.ts
@@ -703,7 +703,12 @@ describe('OperationLogSnapshotService', () => {
       );
     });
 
-    it('should restore backup and not save when migrated state fails validation', async () => {
+    it('should restore backup, not save, but still return the migrated snapshot when state fails validation', async () => {
+      // Non-fatal path: a validation failure here is often reducer-healable (e.g.
+      // an older snapshot missing a newly-required config default). Bricking to
+      // recovery would empty the store on upgrade, so we roll the on-disk cache
+      // back to the backup, never persist the unvalidated snapshot, yet return it
+      // for reducer-healed hydration (Checkpoint C re-checks and persists).
       const snapshot = createSnapshot();
       const migratedSnapshot = { ...snapshot, schemaVersion: CURRENT_SCHEMA_VERSION };
       mockOpLogStore.saveStateCacheBackup.and.resolveTo(undefined);
@@ -714,10 +719,9 @@ describe('OperationLogSnapshotService', () => {
       });
       mockOpLogStore.restoreStateCacheFromBackup.and.resolveTo(undefined);
 
-      await expectAsync(
-        service.migrateSnapshotWithBackup(snapshot),
-      ).toBeRejectedWithError(/Migrated snapshot validation failed/);
+      const result = await service.migrateSnapshotWithBackup(snapshot);
 
+      expect(result).toBe(migratedSnapshot);
       expect(mockOpLogStore.saveStateCache).not.toHaveBeenCalled();
       expect(mockOpLogStore.restoreStateCacheFromBackup).toHaveBeenCalled();
       expect(mockOpLogStore.clearStateCacheBackup).not.toHaveBeenCalled();
diff --git a/src/app/op-log/persistence/operation-log-snapshot.service.ts b/src/app/op-log/persistence/operation-log-snapshot.service.ts
index 4e847f5767..345ce55ce1 100644
--- a/src/app/op-log/persistence/operation-log-snapshot.service.ts
+++ b/src/app/op-log/persistence/operation-log-snapshot.service.ts
@@ -175,9 +175,14 @@ export class OperationLogSnapshotService {
    * Migrates a snapshot with backup safety (A.7.12).
    * Creates a backup before migration and restores it if migration fails.
    *
+   * State-validation failures are non-fatal: the migrated snapshot is returned
+   * for hydration (unpersisted) rather than throwing. See the step-4 note.
+   *
    * @param snapshot - The snapshot to migrate
    * @returns The migrated snapshot
-   * @throws If migration fails and rollback also fails
+   * @throws On migration failure or metadata-validation failure. The backup is
+   *         rolled back first; if that rollback ALSO fails, a combined error is
+   *         thrown instead. State-validation failure does NOT throw (step 4).
    */
   async migrateSnapshotWithBackup(snapshot: StateCache): Promise<StateCache> {
     OpLog.normal(
@@ -197,16 +202,47 @@ export class OperationLogSnapshotService {
         throw new Error('Migrated snapshot metadata validation failed');
       }
 
-      // 4. Validate migrated snapshot state before persisting or clearing the backup.
-      // Otherwise an invalid current-schema cache could be trusted on next startup.
+      // 4. Validate migrated snapshot state before PERSISTING it — an invalid
+      // current-schema cache must not be trusted on next startup (Checkpoint B
+      // skips validation for a matching schema version). But a validation
+      // failure here is NOT fatal to hydration: many "invalid" shapes are
+      // reducer-healable — e.g. a snapshot from an older release missing a
+      // newly-required config field the loadAllData reducer backfills by
+      // default. Bailing to disaster recovery in that case bricks the app to an
+      // empty store on upgrade, even though the data is fine. So on failure we
+      // roll the on-disk cache back to the pre-migration backup (never persist
+      // the unvalidated migrated snapshot) yet still RETURN it, letting the
+      // hydrator load it through the reducer (which heals defaults) and boot
+      // with correct data.
+      //
+      // Caveat: this non-fatal path does NOT itself persist a clean snapshot.
+      // Re-persist only happens if the hydrator's post-replay Checkpoint C runs
+      // and its save gate is met (tail ops present, not a full-state-op load,
+      // >10 replayed ops) — otherwise the on-disk cache stays at the old version
+      // and the migration re-runs every boot (correct, but a startup tax). For
+      // fields with a migration backfill (below) validation passes and step 5
+      // persists a clean snapshot immediately, so that path is the real fix;
+      // this branch is the safety net for the not-yet-backfilled case.
+      // TODO(followup): persist a fresh snapshot after a migration ran whenever
+      // Checkpoint C validates, regardless of tail-op count, to converge in one
+      // boot instead of every boot.
       const validationResult = await this.validateStateService.validateState(
         migratedSnapshot.state as Record<string, unknown>,
       );
       if (!validationResult.isValid) {
-        throw new Error(
-          `Migrated snapshot validation failed (${validationResult.typiaErrors.length} typia errors` +
-            `${validationResult.crossModelError ? `, cross-model: ${validationResult.crossModelError}` : ''})`,
+        OpLog.warn(
+          'OperationLogSnapshotService: Migrated snapshot failed validation — ' +
+            'restoring backup and hydrating unpersisted so the reducer can heal it.',
+          {
+            typiaErrorCount: validationResult.typiaErrors.length,
+            crossModelError: validationResult.crossModelError,
+          },
         );
+        await this.opLogStore.restoreStateCacheFromBackup();
+        OpLog.normal(
+          'OperationLogSnapshotService: Backup restored; returning migrated snapshot for reducer-healed hydration.',
+        );
+        return migratedSnapshot;
       }
 
       // 5. Save migrated snapshot